Planar antennas are antennas that are not adjusted to a specific impedance, but rather require a matching circuit for maximum power transmission. Matching is necessary if the power loss in signal transmission to a non-impedance-matched antenna is to be reduced. Planar antennas, as are used in mobile communications equipment, react by a change of their impedance to changed external effects or proximity to objects such as hands or other body parts of a user, or even metal surfaces.
Connecting a planar antenna to an associated impedance matching circuit is known, for example, from the publication WO 2006/129239 A1. Besides a number of inductive elements, its impedance matching circuit comprises a number of MEMS switches as capacitive elements. The capacitance of a MEMS switch can take two discrete values. A sufficient tuning range for impedance matching is enabled by a plurality of interconnected MEMS switches.
One problem of known impedance matching circuits for planar antennas lies in the fact that either the tuning range is too small or the impedance matching circuit is very complicated and has a large number of interconnected elements. The latter leads to a relatively high susceptibility to defects.